Method for the automated manufacture of pure SO2 from sulfur and oxygen

ABSTRACT

Method for the automated production of liquid SO 2  having a purity above 99.9% from elemental sulfur and pure oxygen, in the presence of SO 2  recirculated from the closed circuit production system itself. The temperature of the sulfur is controlled by means of pre-defined S, O 2  and SO 2  ratios. The automation is made based on oxygen sensors and on sulfur and SO 2  flow meters with their respective control connections and proportional valves permitting a fine control of the sulfur combustion reaction. The liquefaction of the gaseous SO 2  produced is carried out alternatively by a cool plant working between −10 and −60° C. or with a compression unit working at a pressure between 3.8 and 5.0 bar. The automation permits an optimum temperature control and a clean production of SO 2 .

[0001] Obtaining SO₂ from elemental sulfur is a process widely used in the industry. Nevertheless, when a high purity product in industrial amounts and furthermore not polluting the environment is required; the processes fulfilling these requirements are either for small productions, they inevitably produce pollution or they require many purification unit operations, due to the use of air or hydrocarbons as comburent agent or sulfur to oxidize as a liquid.

[0002] The process described in present invention corresponds to an oxidation overcoming the aforementioned general limitations and further carrying out a control of the energy and the purification unit operations better than those standard known ones.

[0003] U.S. Pat. No. 5,204,082 by Schendek (1993) describes a method for preparing high purity SO₂ based on the submerged combustion of elemental sulfur in liquid state, using both air and pure oxygen. This method has the inconvenient of trailing particles of unburned sulfur from the liquid sulfur present both as a reagent and as a solvent. Thus the method uses different unit operations for removing these particles of sulfur trailed from the combustion phase.

[0004] This combustion has the characteristic of having excess of sulfur relative to the reaction air or oxygen, producing a combustion generating side reactions of the sulfur giving reaction by-products; the main purpose of the method is to remove the sulfur from the combustion gases for purifying the SO₂ without considering the gases other than SO₂.

[0005] Therefor it is necessary to carefully control the melted sulfur temperature because the variation of its viscosity with the temperature, in order to avoid the difficulties in the liquid sulfur flow being burnt. In other words, this submerged combustion has the characteristic of requiring a good control of liquid sulfur temperature and its subsequent recycling for obtaining high purity without major concern of gaseous by-products produced together with SO₂.

[0006] U.S. Pat. No. 6,045,770 by Battles (2000) describes the preparation of SO₂ from granulated or emulsified sulfur, by oxidation with air in a sulfur combustion furnace. The reaction from a sulfur emulsion consisting in a suspension of sulfur (70%) in water (30%) with sulfur particles of 4-6 μm is carried out in a sulfur burner atomizing it to sulfur frog or mist for its combustion in a sulfur furnace. The moisture during the combustion generates sulfuric acid as a significant polluting agent from the process.

[0007] As sulfur powder can explode, the combustion with granulated sulfur requires an inert gas such as nitrogen or CO₂ serving as inert medium or diluent agent of the combustion reagents, thus avoiding an eventual lack of control not only of the temperature but also of the combustion products types.

[0008] The processes of submerged combustion and of combustion of atomized sulfur with air, described above, have the characteristic of requiring a safe control of the liquid sulfur temperature due to its abrupt viscosity variations with the temperature; furthermore, the sulfur sublimates and forms crystals when condensing, so as the sulfur storage tanks require to be properly protected against these temperature changes.

[0009] The industrial solid sulfur normally is also accompanied by traces of hydrogen sulfide, H₂S, or of hydrocarbons being constituents of the sulfur source and emitted to the atmosphere transformed after the combustion.

[0010] JP Patent 26011 by Chibooru B. (Bayer AG, 1983) uses the sulfur combustion with oxygen at 1000-1100° C. adding sulfuric acid of 20 to 90% concentration, instead of 800-1300° C. required without adding sulfuric acid. Nevertheless, the purity of the SO₂ produced is not high, specially when for this low temperature it is required further addition of hydrocarbons as fuel compensating the lower temperature.

[0011] JP Patent 309707 by Araki K. (Mitsubishi Heavy Ind. Ltd., 1997) describes an equipment to produce high purity SO₂, based on the combustion of solid sulfur with oxygen or with oxygen enriched air. The combustion gases are cooled with H₂O; the SO₃ is removed with a humid type electric powder collector and a gas separator for removing the unreacted O₂, CO₂ and N₂, in its case. Nevertheless, this system requires various purification steps to obtain a high purity SO₂.

[0012] These patents allow to appreciate that all the described processes require working with the impurities produced in the sulfur oxidation, or combustion, in order to arrive to a process offering pure SO₂.

[0013] Present invention permits to avoid not only the critic control of the oxidation temperature itself, but to simplify the unit operations as the combustion with pure oxygen, diluted with SO₂ produced in the system itself as a closed system, makes its advantages clear for any skill in the art, the thermically autocontrolled oxidation producing high purity SO₂. The automation introduced in this invention allows not only a better control of the process, but also a production of variable capacity.

DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a flowsheet representation showing the system of production of SO₂ from the sulfur feed to the final SO₂ exit and the back of the latter as diluent and cooling agent of the oxidation reaction, with liquefaction by a cooling plant.

[0015]FIG. 2 represents the electronic circuit of the plant automated control showing the oxygen sensors and the flow sensors and their respective control connections.

[0016]FIG. 3 is a schematic diagram of the burner showing the admissions and the distribution of the comburent gas, as primary, secondary and tertiary gas used for the controlled combustion of the atomized sulfur.

[0017]FIG. 4 shows the production plant of SO₂ with the system of liquefaction by compression (15).

DESCRIPTION OF THE INVENTION

[0018] The SO₂ automated production system described in present invention corresponds to the plant automated control based on oxygen sensors and on oxygen and sulfur flow sensors. Furthermore, this description shows how by varying the amounts of liquid SO₂ or final product, the flows of oxygen, sulfur and unliquified return SO₂ vary under control.

[0019] Accordingly, a description of the process carried out for an amount of 5 to 30 tons per day will be provided taking into account that for amounts above 30 tons per day a burner having larger capacity is required.

[0020] The description of the process is made with reference to the general flowsheet shown on FIG. 1. The incorporation of sensors for oxygen, for the sulfur flow and for the SO₂ flow is shown on FIG. 2 specifically representing these components of the process. The features of the combustion are made by reference to FIG. 3 representing the structure of the burner with its atomizer for a production capacity not higher than 30 tons per day. For higher productions only the change of the burner and a combustion chamber suitable for said capacity are required. On the other hand, FIG. 4 represents the liquefaction system of the SO₂ produced by compression as an alternative to the system of liquefaction only by cooling.

[0021]FIG. 2 represents in detail the control diagram of the automated process. In this figure it is shown: the return oxygen sensor, AE/AIT-17, entering together with the return SO₂, measured with the flow sensor FIT 17; the pure oxygen flow sensor, FIT 09; the sulfur flow sensor, FIT 05, as well as the respective control connection.

[0022] Thus, according to the details of FIGS. 1 to 4, a 170 kg/h flow of sulfur in solid or liquid state is entered into the storage tank (1) and then maintained at a temperature of 125-130° C. with the steam produced following the combustion chamber (4) in the heat exchanger (10) of this process. This sulfur enters into the feed tank (2) maintained at a temperature from 130 to 135° C. with the same steam produced after the combustion of the sulfur in the multistep exchanger (10); this steam is the same that, after transferring part of its heat to the feed tank (2) is carried to the storage tank (1). These two steps have the purpose of maintaining the sulfur in liquid state for its handling and for removing the impurities proper of the delivery, in particular the most heavy ones, and the moisture.

[0023] The feed sulfur is maintained liquid in the range of 130-135° C. in the feed tank (2) and therefrom is fed to the atomizer (3), represented in detail in FIG. 3, with a standard fluid pump (5), as at this temperature it offers no problem of viscosity. In this temperature range, the viscosity is lower than 10 mPa/s. As an antecedent, the sulfur under 160° C. rapidly increases its viscosity to reach values of 80,000 mPa/s at 190° C. The liquid sulfur is introduced in the burner (3) being part of the combustion chamber (4).

[0024]FIG. 3 shows in detail the structure of the burner (3). The SO₂ return (17) containing oxygen non consumed from previous oxidation is enter into this burner; pure oxygen (9) is added to this SO₂ before entering into the burner thus forming the fuel with SO₂ as diluent and cooling agent.

[0025] The liquid sulfur is entered (5) through the central back part of the burner (3) and through another also back entrance (6), parallel to the sulfur entrance (5), the O₂ enriched return SO₂ called “primary gas” (6) is entered. The primary gas is the one introduced in wrapping form with the liquid sulfur which is then pulverized by effect of the high speed of the mobile cup of the atomizer in the burner producing microdrops. This mixture of SO₂, oxygen and finely pulverized sulfur in microdrops leaves the rotatory cup of the atomizer and enter into the combustion chamber (4) where the sulfur oxidative combustion step occurs transforming the sulfur in pure SO₂ by the action of oxygen being part of the combustion gas and called tertiary gas (8).

[0026] The rest of the return flow internally wraps the burner acting as cooler and thermally isolating all the front of the burner, and it is called secondary gas (7); in said function, the gas is thus pre-heated in the inner of the combustion chamber (4) wherein it dilutes and cools the total mass produced in the combustion. This is the tertiary gas (8) therewith the temperature does not exceed 1231° C. in the combustion chamber due to the diluting and cooling effect of the SO₂.

[0027] This temperature control in the combustion chamber inner (4) is reached by a suitable handling of the SO₂ return gases (17) by means of the automation based on the return oxygen; therewith a temperature above 1144° C.±50° C. and lower to 1231° C. is reached. In said conditions, the process is easily controlled resulting in a high purity SO₂.

[0028] The combustion gases at the temperature of 1167±50° C. are following passed through a heat multistep exchanger with water (10) shown in FIG. 1, and steam is produced at a temperature range of 145-160° C. mainly used for maintaining in liquid state the feed sulfur (2) entering into the burner (3) and the storage one (1). The steam excess produced can be used for other heat exchange unit operations in an industrial chemical plant.

[0029] The effluent of the sulfur to SO₂ conversion, leaving the multistep heat exchanger at a temperature above or equal to 192° C., is treated in a heat exchanger with atmospheric air (11), so as when leaving it, the combustion gas temperature is 85±5° C.

[0030] The gases pass then into a tower (12) removing SO₃ and the moisture that eventually could enter into the system, with concentrated 98% H₂SO₄ (13) at 30° C. This tower is provided with a sulfuric acid drop trap (14) so as the gases passing therein essentially comprise SO₂ and O₂ unreacted in the combustion chamber.

[0031] The whole mass of gases from the combustion chamber is moved by means of a blower (18) and the mass entering into the cooling plant is moved with the blower (19); the mass returning to the combustion chamber is moved with the blower (20).

[0032] From the total mass moved by the blower (18) up to a 30% is circulated to the cooling plant (15) by means of the blower (19). This cooling plant works at a temperature from −30 to −60° C. From this gas fraction, a high part is liquefied and comprises the SO₂ as final product (16) and the other cooled part comprises essentially SO₂ and O₂; both gases re-enter into the system through the blower (20). This return SO₂ (17) comprising about the 80% of the total SO₂ flow is driven to the combustion chamber (4) but, before entering thereinto it is mixed with the pure oxygen (9) in order to form the comburent mixture entering into the combustion burner.

[0033] Thus the produced SO₂ participates in a closed system or circuit for acting then as diluent and cooling agent of the elemental sulfur oxidation.

[0034] As an alternative to the aforementioned process of SO₂ liquefaction by cooling, the also efficient way of liquefaction by means of a compressor working between 3,8 and 5,0 bar of pressure together with cooling by water allowing an easy controlled liquefaction can be used. This is shown in FIG. 4 wherein it can be appreciated an automated SO₂ production plant producing liquid SO₂ by compression with its compression unit (15) and the lung tank (21). The non compressed gases return to the recycling system through the lung tank, therefrom a blower (20) integrates it to the rest of the SO₂ returning to the combustion chamber (4).

[0035] It must be pointed out that the above described method always works in the absence of nitrogen (the oxidant agent is pure oxygen) different to other industrial systems for the production of SO₂. The storage tank (1) wherein the sulfur is maintained liquefied between 125-130° C., is provided with a funnel with chimney and forced air extraction permitting to evacuate the moisture and the nitrogen present in the sulfur, thus all non desired possible nitrogen impurity is removed in the closed circuit system that the plant has at the moment of burning the sulfur.

[0036] The start point of the process in the plant is initiated with a scavenging with pure oxygen of the whole closed circuit for removing all traces of nitrogen and other gases accompanying the air which eventually could contaminate the liquefied SO₂ production. This sweep with oxygen is thrown to the atmosphere through the cooling plant or the compression plant if that alternative of liquefaction is used, before initiating the combustion process and the SO₂ liquefaction. Thus, the SO₂ production method is free of the presence of nitrogen or other gas different from pure oxygen.

[0037] The chemical analysis of sulfur allows to include as a significant impurity thereof, aromatic polycyclic hydrocarbons, detected by extracting with acetonitrile and by UV an HPLC as standard analysis technique. Analyses of SO₂ produced at the exit of the cooling plant are also carried out, the maximum impurity of SO₂ corresponding to traces of sulfonated polycyclic hydrocarbons; the sulfonation occurs after the sulfur in oxidation passes by the combustion chamber. The analyses show that the impurity does not exceed 0.05%, without discarding the eventual presence of moisture and H₂SO₄ in parts per million.

[0038] As can be appreciated, the control logic is based on the regulation of the O₂ flow at the entrance of the burner based on the S flow and the SO₂ being the cooling and diluent agent of the reaction. The control is carried out by modifying the amount of O₂ so as to have a constant O₂:S ratio in function of the returned O₂ together with the SO₂ with a light excess not exceeding the 5% of the stoichiometric one.

[0039] The flows involved in the control, represented on FIG. 2, are: F05: S flow F09: O₂ flow F17: cooling/diluent flow.

[0040] For measuring the S flow, F 05, a sonic type flow sensor (FE/FIT-05) is used. The O₂ flow, F09, is measured in the same manner and the flow of the returned SO₂ F17 is measured with FIT-09) and FIT-17) rotamers, respectively. The sulfur flow is adjusted by the proportional valve FV.05 and the O₂ is controlled wit a proportional valve FV-07. The content or concentration of O₂ in the F17 flow is measured by a O₂ sensor (AE/AIT-17).

[0041] The first control connection consists in the regulation of the S flow to the burner by the flow meter 05 and the control valve 05.

[0042] The second connection control consists in the regulation of the O₂ flow by the flow meter 09 and the control valve 09. The adjustment of the flow is carried out such as to maintain constant the S:O_(2(total)) ratio. The total O₂ is obtained from the addition of pure O₂ plus the return O₂, the lafter being calculated by the flow sensor 17 and the O₂ analyzer 17.

[0043] The control system allows the adjustment of the oxygen up to a maximum of 5% excess on the stoichiometric one.

[0044] The sulfur oxidation reaction or combustion is previously evaluated in order to know the amounts and proportions of the reactants to be mixed for their reaction; this is carried out with the HSC reaction simulated program of Outokumpu^((TM)). This program provides the reaction temperature reached in the combustion chamber for different O₂, S and SO₂ mass ratios of the oxidation closed circuit in different mixtures.

[0045] For example, for:

S+(O₂+SO₂)→SO₂

[0046] if the reactants and return SO₂ masses are that following indicated, the predicted reaction temperature is the included in the fourth column of the following Table. S (g) O₂ (g) SO₂ (g) T (° C.) 32.00 32.64 0 5018.8 32.00 33.60 0 5018.7 32.00 32.63 243.42 1211.56 32.00 33.60 243.42 1211.53 32.00 32.63 256.23 1167.57 32.00 33.60 256.23 1167.55 32.00 32.63 262.64 1146.88 32.00 33.60 262.64 1146.75

[0047] The temperature reached in the combustion chamber mainly depends on the S:O₂:SO₂ ratio and not so much on the temperature of the fuel entering into the burner; if the temperature of the SO₂ entering into the burner is variable, the reaction temperature is the one indicated for the ratio, expressed in grams, as follows:

S:O₂SO₂=32:33,6:256,2

[0048] Temperature of the return SO₂ Reaction temperature  0° C. 1164.77° C. 15° C. 1167.55° C. 30° C. 1175.93° C. 50° C. 1187.28° C.

[0049] The above description with the features of the oxidation process itself without gases generating byproducts, such as nitrogen, ang only with pure oxygen diluted with SO₂ produced in the closed circuit system; the SO₂ high purity controlled by analysis before and after the combustion; its stepping for different production capacities; the different routes for obtaining liquid SO₂ either by cooling or compression; as well as the oxidation process computer simulation indicate for any skill in this area of processes that the system offers significant advantages relating to the state of the technique not automated and not working under closed circuit such as the one described in present patent application.

[0050] The automated control of the flows of sulfur and oxygen carried out based on the return oxygen, is the base of the efficiency of the process for obtaining high purity liquefied SO₂ in safe manner free of any environmental contamination. 

1. A method for the automated production of pure SO₂ from elemental sulfur and oxygen in the presence of SO₂ CHARACTERIZED in that it is a completely regulated combustion system.
 2. A method for the production of SO₂ according to claim 1 CHARACTERIZED in that the SO₂ is used as cooling and diluent agent of the reactants and comprises a fraction of the SO₂ produced in the sulfur oxidation.
 3. A method for the production of SO₂ according to claim 1 CHARACTERIZED in that the oxygen not consumed during the combustion is re-entered into the sulfur combustion chamber together with the return SO₂.
 4. A method for the production of SO₂ according to claims 1, 2 and 3 CHARACTERIZED in that the temperature of the sulfur combustion is controlled by maintaining a defined ratio of S, O₂ and SO₂.
 5. A method for the production of SO₂ according to claim 4 CHARACTERIZED in that the S:O2:SO2 ratio expressed in grams, entering into the combustion chamber is regulated in the range from 32:32.63:243.42 to 32:33.6:262.64 and more frequently of 32:32.63:256.23.
 6. A method for the production of SO₂ according to claims 1 to 5 CHARACTERIZED in that the regulation of oxygen entering into the combustion chamber is carried out based on an on line sensor of the oxygen returning to the combustion chamber after the steps of purification and cooling, of the SO₂ produced in closed circuit.
 7. A method for the production of SO₂ according to claim 6 CHARACTERIZED in that the regulation of the pure oxygen added to the return oxygen before entering into the combustion chamber is carried out based on a proportional valve controlled by the return oxygen sensor.
 8. A method for the production of SO₂ according to claim 7 CHARACTERIZED in that the automated regulation system allows a control of the oxygen entering into the combustion chamber of a 2%-5% excess relating to the stoichiometric value of S and O₂.
 9. A method for the production of SO₂ according to claims 1 to 5 CHARACTERIZED in that the required amount of sulfur is controlled based on a sulfur flow sensor.
 10. A method for the production of SO₂ according to claim 9 CHARACTERIZED in that according to the desired final SO₂ production the entrance of sulfur to the chamber combustion is regulated by a proportional valve controlled by a return oxygen sensor.
 11. A method for the production of SO₂ according to claim 10 CHARACTERIZED in that the sulfur in liquid state is entered into the combustion chamber at a temperature between 130 and 135° C. maintained by a steam produced in a multistep heat exchanger post-combustion chamber.
 12. A method for the production of SO₂ according to claims 1 and 11 CHARACTERIZED in that the combustion is produced from liquid elemental sulfur in pulverized microdrop state produced in the burner combustion atomizer.
 13. A method for the production of SO₂ according to claims 1 to 12 CHARACTERIZED in that the combustion chamber is maintained at a mean temperature of 1167,5°, preferably above 1100° C. and under 1250° C. and most preferably at 1160° C.±50° C.
 14. A method for the production of SO₂ according to claim 13 CHARACTERIZED in that the formed SO₂ contains small amounts of SO₃ which are absorbed countercurrent in a 98% H₂SO₄ tower.
 15. A method for the production of SO₂ according to claim 13 CHARACTERIZED in that a fraction of up to 30% of the dry produced SO₂ and remaining oxygen are passed to a cool plant working between −10 and −60° C., a high part of the SO₂ comprising the final liquid SO₂ being liquefied.
 16. A method for the production of SO₂ according to claim 13 CHARACTERIZED in that a fraction up to 30% of the dry generated SO₂ and remaining oxygen alternatively enter into a compression liquefaction unit working at a pressure between 3,8 and 5,0 bar and water cooler working under 32° C., allowing the liquefaction of a great part of the SO₂ comprising the final liquid SO₂.
 17. A method for the production of SO₂ according to claims 8 and 15 or 16 CHARACTERIZED in that the capacity of liquefaction of SO₂ is favored by the absence of an uncontrolled excess of oxygen mass and a higher concentration of gaseous SO₂.
 18. A method for the production of SO₂ according to claim 15 or 16 CHARACTERIZED in that up to 70% of the SO₂ not passing through the cooling plant, is sent back as cooling and diluent agent to the burner of the sulfur combustion chamber, previously to mixing it with unliquefied gas SO₂, further to the remaining unreacted oxygen.
 19. A method for the production of SO₂ according to claims 1 to 18 CHARACTERIZED in that the SO₂ produced has a purity above 99,90%.
 20. A method for the production of SO₂ according to claim 19 CHARACTERIZED in that the elemental sulfur content is under 2 ppm.
 21. A method for the production of SO₂ according to claims 19 and 20 CHARACTERIZED in that the greatest impurity detected en the final SO₂ corresponds to polycyclic aromatic hydrocarbons contained in the original sulfur.
 22. A method for the production of SO₂ according to claim 21 CHARACTERIZED in that the aromatic compounds of the impurities are sulfonated after the sulfur oxidation.
 23. A method for the production of SO₂ according to previous claims CHARACTERIZED in that the whole process is not environmental contaminant and energetically favored.
 24. A method for the production of SO₂ according to claim 23 CHARACTERIZED in that the automated combustion control permits working in optimal conditions for the required proportion of the reactants and the safety process.
 25. A method for the production of SO₂ according to claim 24 CHARACTERIZED in that the controlled energy of the process allows a higher durability of the associated equipment.
 26. A method for the production of SO₂ according to claim 24 CHARACTERIZED in making possible a more rational use of the energy as a mean of generating steam either for the process itself and for other processes complementary of the production plant. 